1. Field of the Invention
The present invention relates to an apparatus for controlling an internal combustion engine which emits exhaust gases through a catalytic converter.
2. Description of the Related Art
Catalytic converters such as three-way catalytic converters are disposed in the exhaust systems of internal combustion engines for purifying exhaust gases emitted from the internal combustion engines. Generally, such a catalytic converter cannot maintain a desired ability to purify exhaust gases unless its temperature rises to a level high enough to activate the catalytic converter. It is, therefore, important for the catalytic converter to achieve a desired level of exhaust gas purification immediately after the internal combustion engine has started to operate when the temperature of the catalytic converter is comparatively low.
The applicant of the present application has proposed a technology for purifying exhaust gases when the catalytic converter has a relatively low temperature, as disclosed in Japanese laid-open patent publication No. 10-299631 and U.S. patent application Ser. No. 09/063732. The proposed exhaust gas purifying system will be described below.
When an internal combustion engine idles after it has started to operate, the amount of intake air introduced into the internal combustion engine is made greater than when the internal combustion engine idles normally, e.g., when the internal combustion engine idles after an automobile propelled by the internal combustion engine has traveled. Furthermore, after the amount of intake air has started increasing, a command value for the ignition timing of the internal combustion engine is generated according to a feedback control process (specifically, a PI (proportional plus integral) control process) in order to converge the rotational speed (actual rotational speed) of the internal combustion engine, which tends to increase due to the increasing amount of intake air, toward a certain target rotational speed. The actual ignition timing of the internal combustion engine is then controlled based on the generated command value, so that the ignition timing will be more retarded than normal.
When the amount of intake air is increased and the ignition timing is retarded in the manner described above, the exhaust gases emitted by the internal combustion engine upon combustion of the air-fuel mixture have a greater amount of heat, and hence the catalytic converter heated by the exhaust gases is activated quickly. As a result, the catalytic converter is capable of achieving a desired purification ability quickly after the internal combustion engine has started to operate.
According to the above proposed technology, when the rotational speed of the internal combustion engine, which tends to increase due to the increasing amount of intake air, is controlled so as to converge to the target rotational speed according to the feedback control process (hereinafter referred to as an "ignition timing control rotational speed F/B control process"), the ignition timing is controlled so as to be retarded while the rotational speed of the internal combustion engine is being maintained stably. Therefore, the amount of intake air can be increased and the ignition timing can be controlled independently of each other. In addition, the system for controlling the amount of intake air and the ignition timing can easily be constructed and simplified.
The feedback gain of the ignition timing control rotational speed F/B control process, i.e., the rate of change of the command value for the ignition timing to the difference between the actual and target rotational speeds of the internal combustion engine (hereinafter referred to as a "rotational speed difference"), has heretofore been set to a feedback gain fixed with respect to the ignition timing, which has been determined by way of experimentation and simulation. Such feedback gain determination is equivalent in the PI control process to the determination of coefficient parameters relative to proportional and integral terms (gains of proportional and integral terms) as fixed values with respect to the ignition timing.
The inventors of the present application have found as a result of further studies that the rotational speed of the internal combustion engine may possibly become unstable with respect to the target rotational speed according to the conventional technology.
Various studies made by the inventors of the present application have revealed that the rate of a change of the rotational speed (actual speed) of the internal combustion engine to a change of the ignition timing tends to be greater as the ignition timing is more retarded. When a command value for the ignition timing is generated to converge the rotational speed of the internal combustion engine to the target rotational speed according to the ignition timing control rotational speed F/B control process, the command value is affected various operating conditions of the internal combustion engine. Consequently, the command value for the ignition timing and the actual ignition timing controlled depending on the command value often become relatively largely retarded.
When the ignition timing of the internal combustion engine is relatively largely retarded, a change of the command value for the ignition timing generated depending on the rotational speed difference becomes excessively large, meaning that the feedback gain of the ignition timing control rotational speed F/B control process is excessively large. As a result, the actual rotational speed of the internal combustion engine excessively changes to the target rotational speed, and the actual rotational speed becomes unstable (the actual rotational speed fluctuates in an oscillatory fashion with respect to the target rotational speed).
One solution would be to set the feedback gain of the ignition timing control rotational speed F/B control process to a relatively small gain. With such a relatively small feedback gain, however, insofar as the ignition timing has not been largely retarded, the quick response of the control process for converging the rotational speed of the internal combustion engine to the target rotational speed according to the ignition timing control rotational speed F/B control process is reduced.